1. Field of the Invention
The present invention generally relates to the technology of electron beam lithography, and more specifically to a method for proximity effect correction in electron beam lithography systems.
2. Description of the Prior Art
In recent years, various electron beam lithography systems have been employed to write fine patterns on samples such as semiconductor wafers. The samples are typically covered by a thin film called a resist. The resist properties are modified by exposure to an electron beam according to a desired pattern. The patterned resist film is used as a stencil for subsequent processing. Electrons emitted towards the sample collide with nuclei, etc., of the sample and scatter. This is called backward scattering. A phenomenon called forward scattering also occurs. Forward scattering is the electron scattering that occurs in the resist prior to entry into the substrate. Forward scattering also represents the finite beam spread in real electron beam systems. This scattering of electrons may result in distortions to the desired pattern. This is called the proximity effect. The proximity effect may change the shape and dimensions of the part of the sample under the resist from those required.
Various correction techniques to reduce the proximity effect have been developed. The most accurate techniques compute the effects of electron scattering on design patterns on a shape-to-shape basis, varying the electron dose applied to an individual shape, such that all shapes in a pattern receive the same total absorbed electron energy (i.e., the exposure). This is known as an exposure equalization technique.
An increase in the number of devices and elements on a chip increases the chip density and complexity. The chip density and complexity is further increased as features become smaller in size. This translates into greater demands on the computing resources required for performing proximity correction, as more shape-to-shape interactions must be computed. In addition, new electron beam lithography tools are now available which operate at high incident electron energies (approximately 100 Kev), increasing the range of back scattered electrons, resulting in an increase in the number of shape-to-shape interactions which must be computed.
Common chip design tools employ a powerful data compaction technique, known as nesting or design hierarchy, whereby a shape or a collection of shapes (i.e., cell) may be replicated any number of times in different parts of the design pattern without designing new shapes each time. This is done by simply referring to the original shape (or cell) for each subsequent placement, or instance, of that shape (or cell).
In order to perform accurate proximity correction, it is necessary to know precisely the effects of all the shapes within the scattering range of a given shape. For a 100 KeV electron beam, this range may extend as far as 50 .mu.m on a silicon substrate. Thus, the calculation of shape-to-shape interactions must be done for a large number of shapes. In general, this requires the loss of data compaction given by efficient use of design hierarchy, since the scattering effects influencing a given shape in a design cell may vary from one instance of the shape (or cell) to another.
In U.S. Pat. No. 5,051,598 issued Sep. 24, 1991, to Ashton et al., entitled, METHOD FOR CORRECTING PROXIMITY EFFECTS IN ELECTRON BEAM LITHOGRAPHY, incorporated herein by reference, a technique is described which provides a significant improvement in proximity correction efficiency, referred to as the dose-on-grid technique. This technique is based on the splitting of the proximity function into two separate portions: a local portion, describing the effects due primarily to forward scattering of the incident electron beam; and a remote portion, describing the effects due to long range back scatter of electrons.
The back scatter portion is generally a more slowly varying function of distance from the incident electron beam than is the local portion, and it may be accurately represented on a grid. According to the technique disclosed in Ashton et al., a fractional density matrix is generated. This matrix represents the pattern shape density on the grid, without reference to the individual shapes. A proximity matrix is also computed, which represents the back scatter (remote) portion of the proximity function under a set of given electron beam exposure conditions (i.e., electron energy, substrate composition). When the density matrix is convolved with the proximity matrix, the resulting dose matrix, represents the back scatter corrected dose field on the grid. Individual shape doses are computed by interpolating the dose field from nearby grid points onto the shape.
Despite the significant computational advantages offered by the dose-on-grid technique, the data compaction of the original design hierarchy is lost, since, in generating the fractional density matrix, the positions of all shapes in the pattern must be known. The result is twofold: (a) data storage volumes may increase dramatically as the fractional density matrix is generated by replacing individual shapes, and (b) in the calculation of the fractional density matrix, the same calculation may be repeated on the same shapes numerous times, resulting in inefficiency.
For the foregoing reasons, there is a need for a method for efficiently computing a dose correction in electron beam lithography systems whereby the pattern design hierarchy is retained to as great an extent as possible during the process of proximity correction.